This invention relates to a semiconductor device, and more particularly to a power transistor adapted to a high power semiconductor integrated circuit device (hereinafter referred to as a "power IC").
In the power transistor in a power IC for use in an audio output circuit or the like, it is customary practice to divide an emitter region into a plurality of regions and arrange them in one base region in order to obtain a high output current. In such a power transistor, a ballast resistance is inserted between each emitter region and the one emitter input terminal so that the emitter current flows uniformly through the plurality of divided emitter regions. As a method of forming this ballast resistance, it is conceivable in principle to form a resistance region with one end continuous with the emitter regions and the other end connected to the emitter input electrode. This would reduce the area occupied by the power transistor. However, according to the results of experiments carried out for such a construction by the inventor of the present invention, it has been found that the input electrode side of the resistance region and the resistance region function as transistors. As a result of this, the transistor formed by the emitter regions of the original transistor and the parasitic transistor formed by the resistance region exist together in the substrate. This leads to local current concentration in the parasitic transistor due to the ballast resistance region, which, in turn, leads to a breakdown of the transistor.